1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to a liquid crystal medium composition of liquid crystal display (LCD).
2. The Related Arts
In the liquid crystal display industry, the recently developed technique of polymer stabilized vertical alignment (PSVA) has various advantages, such as wide view angle, high contrast, and fast response, over the traditional twisted nematic/super twisted nematic (TN/STN) liquid crystal displaying techniques and also shows superiority to other vertical alignment (VA) techniques, such as multi-domain vertical alignment (MVA) and patterned vertical alignment (PVA) in respect to transmittance and simplification of manufacture process. Consequently, PSVA is the technical main stream of the contemporary thin-film transistor (TFT) LCD industry.
The key manufacture process of PSVA is illustrated in FIG. 1. PSVA techniques use negative liquid crystal material, in which when no electrical field is applied, the liquid crystal molecules 106 are arranged to perpendicular to substrate surfaces of upper and lower glass substrates 102, 100. ITO slits 101 are formed on a TFT side of the lower glass substrate 100, while ITO 104 of the upper glass substrate 102 remains intact. Monomer 108 that is referred to as reactive monomer (RM) is added in the liquid crystal material to carry out polymerization under irradiation of ultraviolet light. After the liquid crystal panel is assembled through dropping of liquid crystal therein, a signal of proper frequency, waveform, and voltage can be applied to the panel to have the liquid crystal molecules tilting in a predetermined direction, and at the same time, ultraviolet light is applied to irradiate the panel so as to polymerize the reactive monomers to form polymer deposited on the surface of the upper and lower glass substrates 102, 100 of the panel. The polymerized polymer deposit forms polymer bumps 110 that make the liquid crystal molecules 106 showing a predetermined tilt angle without application of electrical voltage thereto, whereby the response of the liquid crystal molecules are speeded up. An advantage of this technique is that multi-angle alignment (generating pre-tilt angle) can be realized in the panel.
The photo-reactive groups contained in the reactive monomers that most commonly used in the state-of-the-art PSVA technique are often methacrylate group, ethenyl group, ethyleneoxy group, or epoxy group, among which the commonly used one is methacrylate group. The wavelength that may cause photo polymerization of reactive monomers that contain such groups are of a range of 200-300 nm. Ultraviolet light having a wavelength exceeding 300 nm may cause the reaction of the reactive monomers, but the efficiency is low and speed is slow, making it showing no excellent productivity. Therefore, irradiation of the panel must be done with a light source having a wavelength lower than 300 nm in order to make the reactive monomers react. However, using a light source having a wavelength less than 300 nm brings certain drawbacks and troubles to the manufacture of the panel. Firstly, ultraviolet light with a wavelength less than 300 nm has a more intense energy, which may cause degradation and damage of polyimide that is the material of the alignment layer and VA liquid crystal molecules adopted in the technique, making voltage holding ratio (VHR) of the panel lowered, image sticking becoming severe, and reliability analysis (RA) poor. Further, the glass that is used to make the upper and lower glass substrates of an LCD generally shows absorbability of ultraviolet light having a wavelength less than 300 nm. As illustrated in the transmission spectrum of liquid crystal with respect to ultraviolet light shown in FIG. 2, it is noted that ultraviolet light with a wavelength less than 300 is completely blocked from transmitting through the liquid crystal material. In other words, most of the ultraviolet light from the light source is absorbed by the liquid crystal material (thus causing an effect of damage) and only a minor portion is absorbed by the reactive monomer to induce the polymerization reaction. This occurs at a very shallow position at the light incidence side, and thus non-uniformity of reaction of the reactive monomers at the light incidence side and the side away from light may result, leading to deteriorated effect of alignment for the panel.